Referring to FIG. 1, it is shown that a charge pump circuit is conventionally used for a current mirror to drive a light emitting diode (LED). Charges are stored and transmitted depending on the one-way transmission of the diode-connected MOS and the capacitance, and therefore a high voltage is formed and output. The conventional charge pump circuit is formed by connecting several pump circuit units 11 in series. Please refer to FIG. 2. Each pump circuit unit 11 is formed by coupling a first capacitor C22, a first transistor N23, a second capacitor C25 and a second transistor N26, wherein the first transistor N23 is connected to the second transistor N26 to form a common drain, then the common drain is connected to one end of the first capacitor C22 and one end of the second capacitor C25, respectively.
However, there are two drawbacks as follows when a light emitting diode is driven by the conventional charge pump circuit.
1. Referring to FIG. 1, the charges are stored by the capacitor of the charge pump circuit. The maximal efficacy of a secondary charge pump circuit is Vout/(2 Vcc), even Vout/(1.5 Vcc). It means that the efficiency of the charge pumping is determined by the input voltage Vcc. Therefore, the efficiency of the charge pump circuit can not be improved by improving the applied circuits or improving the peripheral elements.
2. Please refer to FIG. 3 illustrating a circuit of a white light emitting diode driven by a current mirror in the prior art. The input voltage of the current mirror constructed by the transistors N31, N32, N33 and N34 is corresponding to the output voltage Vout of the charge pump circuit shown in FIG. 1. Since the current mirror is controlled by the charge pump circuit, wherein Vout is fixed at 5V and the input voltage VF of the white light emitting diode D35 is in a range from 3.1V to 3.8V, excess power loss occurs on the current mirror. It can be illustrateed as the equation (1).PLEDLOSS=ILED(VOUT−VF)  (1)
In order to overcome the foresaid drawback of the prior art, the present invention provides a method for driving a light emitting diode with a current mirror.